1)
What is the meaning of "v'Hayah Im Zara Yisrael"?
1.
Radak: It is when they would seed. This is like "v'Hayah Im Ba El Eshes Achiv" (Bereishis 38:9), "v'Im Yihyeh ha'Yovel" (Bamidbar 36:4).
2.
Chomas Anach: Most people refrained from sowing, due to the affliction of Midyan. If an individual sowed (Midyan came); the singular hints to this .
2)
What is the reason for the repetition "v'Alah Midyan... v'Alu Alav... va'Yachanu Aleihem" (verse 4)?
1.
Malbim: V'Alu Alav refer to the crops; va'Yachanu Aleihem is against Yisrael themselves.